The role of vitamin C in supporting normal immune function and reducing the impact of upper respiratory tract infections

Background

Vitamin C, also called ascorbic acid, is a water-soluble, anti-oxidant vitamin present in foods including citrus fruits, citrus and other fruit juices, tomatoes, red and green peppers, kiwi fruit, strawberries, broccoli and potatoes.

The vitamin C level in plasma of people in good health becomes saturated at about 70 micromol/L when the intake is about 200mg/day[1]. Below intakes of 100mg daily there is a steep relationship between dose and plasma concentration. This suggests that intakes/supplementation above 200mg/day would bring no benefit.

However, some studies have shown benefits of supplementation even in the context of a regular background intake of 500mg daily. If regular intakes are below 100mg/day, benefits could be expected from higher intakes on the basis of the intake/plasma concentration curve. However, infection itself causes altered vitamin C metabolism and vitamin C plasma levels may be decreased in cases of infection.

Potential mechanisms for an impact of vitamin C on immune function

Vitamin C levels in white blood cells are 10-100 times higher than in plasma[2], and these cells can accumulate vitamin C against a concentration gradient[3] which may indicate functional roles of the vitamin in these immune system cells.

Vitamin C has been shown to affect the functions of immune cells such as neutrophils which are types of phagocytes[4]. Laboratory studies reveal that vitamin C can influence the production of interferon (a protein produced by immune cells which can inhibit virus replication), actively slow the replication of viruses, and promote the maturation of T-lymphocytes[5][6][7][8]. Vitamin C also displays anti-inflammatory activity[9][10]

As vitamin C is an antioxidant, its effects may be most prominent when oxidative stress is increased. ‘Oxidant-antioxidant’ balance is an important determinant of immune function, and immune cells are particularly sensitive to changes in this balance because of the higher percentage of polyunsaturated fatty acids in their plasma membranes[11]. Oxidative damage can lead to a loss of membrane integrity and altered membrane fluidity, adversely affecting the transmission of signals both within cells, and between different immune cells.

Many infections activate the phagocytes, a type of immune cell. A consequence of this is increased production of reactive oxygen species (ROS) which have a role in processes that deactivate viruses and kill bacteria[12]. However, many of the ROS can also be harmful to host cells (cells inside which a virus or bacteria replicates) and, in some cases, could exacerbate infections in the short term[13][14].

Since vitamin C has powerful antioxidant effects, it can protect host cells against the actions of ROS released by phagocytes in response to infection. Phagocytes (including neutrophils) have a specific transport system by which the oxidized form of vitamin C (dehydroascorbic acid) is imported into the cell where it is then converted into an active form of vitamin C[15] .

Potential mechanisms for the impact of vitamin C as an anti-viral agent

To recap, vitamin C’s potential role as an anti-viral substance stems from its antioxidant activity and positive effects on immune cells. Vitamin C also has a role in collagen synthesis which is needed to stabilise epithelial barriers (the cell barriers protecting the respiratory tract, skin and gut).

A laboratory study found that ascorbic acid inhibited the multiplication of viruses of three different families: herpes simplex virus type 1 (HSV-1), influenza virus type A and poliovirus[16], although the effects were modest.

Another laboratory study found that a combination of red ginseng and vitamin C enhanced the activation of T and NK cells (immune cells) and repressed the viral lytic cycle (the stage where the virus bursts out of the host cell to infect other cells). Vitamin C also reduced lung inflammation caused by viral infection[17].

In a further laboratory study, vitamin C caused an anti-viral immune response through the production of Interferon during the early stages of the influenza A virus[18].

Conclusions and implications for 100% fruit juice

In summary, the literature suggests than a regular intake of vitamin C of up to 200mg daily might be helpful for immune function. There is no specific literature on 100% fruit juice and immune function but orange juice, for example, contains approximately 45mg vitamin C per 100g, or 90mg per 200mL glass making it an easy way to achieve the European Nutrient Reference Value of 80mg.

Given that disease treatment or prevention claims cannot be made within the Nutrition and Health Claims regulations, it is recommended that communications around vitamin C are based on authorised article 13 claims (e.g. vitamin C supports normal immune function) and research evidence on the mechanisms which explain how vitamin C supports the body’s immune response to upper respiratory tract viruses.

Glossary of terminology

Antioxidant: a substance that can prevent or slow damage to cells caused by free radicals i.e. unstable substances that the body produces in response to environmental or other pressures such as infections, smoking, pollution, unhealthy diets.

Cytokines: a broad category of proteins released by white cells; they are involved in sending signals to cells to regulate immunity

Interferons: a group of proteins within the category of cytokines made and released by host cells (human or animal) in response to the presence of viruses. Interferon work by causing nearby cells to increase their anti-viral activity.

NK cells: natural killer cells, a subset of T cells.

Oxidant/antioxidant balance: is vital for healthy tissues. A disruption of this balance causes tissue damage, known in this context as ‘oxidative stress’. Oxidant/antioxidant balance is maintained by consuming antioxidants from the diet e.g. vitamin C, polyphenols.

Reactive oxygen species (ROS): chemically reactive species derived from oxygen. These are produced during normal metabolic functions in the body but are also created in response to external stimuli e.g. stress, pollution, smoking.

T cells: also called T lymphocytes, a type of white blood cell that is an essential part of the immune system.

Virus: a sub-microscopic infectious agent that replicates only inside the living cells of an organism, called the ‘host’.

[2]
Micronutrients at the Interface Between Inflammation and Infection Ascorbic Acid and Calciferol. Part 1: General Overview with a Focus on Ascorbic Acid Alexander Ströhle* , Maike Wolters and Andreas Hahn

[4]
Phagocytes are a group of cells that protects the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Phagocytes include many types of white blood cells (such as neutrophils, monocytes, macrophages, mast cells, and dendritic ce